Modular dc-dc converter

ABSTRACT

A modular dc-dc boost converter system is provided that can substantially improve efficiency over a wide range of input and output voltages. The system includes three modules: a buck module, a boost module, and a dc transformer module. These modules are interconnected such that the system output voltage is equal to the sum of the output voltages of adc-dc converter module and a dc transformer module. Depending on the operating point, one or more modules may operate in passthrough mode, leading to substantially reduced ac losses. The required capacitor size and the transistor voltage ratings are also substantially reduced, relative to a conventional single dc-dc boost converter operating at the same input and output voltages.

FIELD

The present disclosure relates to DC-DC power conversion thatincorporating a DC transformer and converts an input voltage to anoutput voltage.

BACKGROUND

A modular dc-dc conversion system to boost a voltage is disclosed in thefollowing patent application:

-   Robert Erickson, “Integrated Photovoltaic Module” U.S. patent    application Ser. No. 13/318,589, May 10, 2010, which is incorporated    by reference herein in its entirety as if fully set forth herein.    The approach in this reference provides a non-inverting buck-boost    converter arranged in series with a unidirectional DC    transformer (DCX) module. The reference does not disclose a DCX    module whose output port is connected in series with the output of a    converter module, such as a boost module.

A publication that provides a detailed analysis of a DC transformercircuit, such as the DCX circuit shown in FIG. 8, including designinformation, is the following:

-   D. Jones and R. Erickson, “Analysis of Switching Circuits through    Incorporation of a Generalized Diode Reverse Recovery Model into    State Plane Analysis,” IEEE Transactions on Circuits and Systems I,    vol. 60, no. 2, pp. 479-490, February 2013, which is incorporated by    reference herein in its entirety as if fully set forth herein.

A publication that describes a method for controlling buck and boostconverters using pass-through modes is the following:

-   D. Jones and R. Erickson, “A Nonlinear State Machine for Dead Zone    Avoidance and Mitigation in a Synchronous Noninverting Buck-Boost    Converter,” IEEE Transactions on Power Electronics, vol. 28, no. 1,    pp. 467-480, January 2013, which is incorporated by reference herein    in its entirety as if fully set forth herein.

A DC-DC boost converter increases a DC input voltage V_(in) to produce aDC output voltage V_(out)=MV_(in), where the conversion ratio M isgreater than or equal to one. An example of a well-known implementationof a boost converter 10 is illustrated in FIG. 1. In this circuit 10, acontroller circuit drives a transistor gate 12 with a repetitive signalthat causes the transistor Q to be ON for a time DT_(s), and OFF for atime (1−D)T_(s), where D is the transistor duty cycle and T_(s) is theswitching period. When the transistor Q is ON, energy from an inputsource is stored in the inductor L. When the transistor Q is OFF, thediode D becomes forward-biased by an inductor current, and energy storedin the inductor L is released to the output. To the extent that thecircuit elements have low power loss, the output voltage is given byV_(out)=V_(in)/(1−D), and the efficiency η=P_(out)/P_(in) can approach100%. A bi-directional converter 20 that is an extension of theconventional DC-DC boost converter is illustrated in FIG. 2, in which apair of transistors Q1 and Q2 and a pair of diodes D1 and D2 allow theinductor current to be either positive or negative, so that power canflow from either V_(in) to V_(out) or V_(out) to V_(in).

It is well known that a variety of loss mechanisms reduce the efficiencyof the boost converters of FIG. 1 and FIG. 2. These loss mechanisms canbe broadly grouped into DC losses and AC losses. In this disclosure, DClosses refer to losses that do not depend directly on the switchingfrequency, such as losses arising from the forward voltage drops of thesemiconductor devices and losses caused by the DC resistance of theinductor winding. AC losses refer to losses that increase with switchingfrequency, such as semiconductor switching losses caused by transistorand diode switching times, diode reverse recovery, semiconductor outputcapacitances, and transistor drive power. The inductor also exhibits AClosses caused by core loss as well as AC winding losses arising from theskin and proximity effects. As a result of these loss mechanisms, theconventional boost converter circuit may exhibit substantially degradedefficiency. Furthermore, the efficiency is a function of input andoutput voltage, switching frequency, and output power. FIG. 3illustrates typical efficiency curves of a boost converter, for severalvalues of resistive load. It can be seen that the efficiency degrades asthe duty cycle (and hence also output voltage) is increased.

Typically, power converters are thermally limited by their coolingsystems, and these cooling systems may have significant size and cost.For a given cooling technology and cooling system size, there is a fixedamount of loss that can be tolerated while maintaining an acceptabletemperature rise. In a thermally limited system, improvement ofefficiency means that the output power can be increased. For example, ifthe efficiency can be increased from 96% to 98%, then the loss isapproximately halved. Assuming that the system is stillthermally-limited and the cooling system size is maintained constant,then the rated output power can be doubled and the cost per watt ofoutput power is halved. Ultimately, it is desirable to increase theratio P_(out)/P_(loss) so that the converter cost per watt, or coolingsystem size and cost, are decreased.

A conventional boost converter also exhibits reduced efficiency at lowoutput power, as a result of AC losses. Converter efficiency over arange of output powers and voltages is increasingly important becausethe converter may operate at partial power for a substantial fraction ofthe time. For example, power converters for solar power systems arecharacterized by a weighted efficiency that accounts for efficiency notonly at rated power, but also at lower powers corresponding to less thanfull irradiance. Power converters for electric vehicle applications mustoperate over driving profiles having a wide variety of speeds andaccelerations, corresponding to a variety of converter output voltagesand powers; improvement of efficiency at all of these operating pointsis needed to improve the effective miles per gallon (MPGe) of thevehicle. Power converters for grid interface of wind turbines must alsooperate efficiently with a wide range of voltages and output powers,corresponding to a range of wind speeds. FIG. 4 illustrates a typicalefficiency curve for a conventional boost converter, operating at aconstant output voltage and with variable output power. It is desirableto increase the efficiency not just at maximum power, but also at lowerpowers.

AC switching losses can be reduced by reduction of the switchingfrequency. However, this necessitates use of larger inductor andcapacitor elements, which are more expensive. The larger inductor mayalso exhibit higher DC resistance. Therefore, it is often undesirable toreduce the switching frequency, and solutions are needed that achievehigh efficiency without sacrificing switching frequency.

The size of the output capacitor is often limited by itsroot-mean-square (RMS) current rating. The RMS capacitor currentincreases as the duty cycle is increased. To reduce the size and cost ofthis capacitor, an improved circuit is needed that can boost the voltagesubstantially, while maintaining relatively low RMS capacitor current.

High-voltage power semiconductor devices typically exhibit increasedswitching times and increased switching losses. In a boost convertersystem, for example, it may be desirable to avoid use of high-voltagesemiconductor devices, employing multiple lower-voltage devices instead.A well-known example of this is a multilevel converter; a three-levelboost converter is illustrated in FIG. 5. This converter circuit 30 canachieve some of the goals delineated here, including reduction of AClosses and use of semiconductors with reduced voltage rating. However,it operates with substantially increased capacitor RMS currents, andhence requires expensive capacitors.

SUMMARY

In various implementations, for example, a modular DC-DC converter isprovided that employs semiconductors with reduced voltage ratings, whilealso reducing the capacitor RMS currents.

In one implementation, DC-DC power conversion incorporating a DCtransformer that converts an input voltage to an output voltage isprovided. In various implementations, a modular DC-DC power conversionis provided to improve converter efficiency over a wide range ofconversion ratios and output powers. In one particular implementation, amodular architecture includes a DC transformer (DCX) module and at leastone converter module capable of being operated in a pass-through mode.For example, a modular architecture may include a DC transformer moduleand at least one of a boost converter module, a buck converter moduleand a non-inverting buck-boost converter module. The modules may beconfigured as and controlled such that efficiency is improved.

In one implementation, a boost DC-DC converter improves the efficiencyof a DC-DC boost converter system, through reduction of the AC losses;improves converter efficiency over a range of operating points, i.e., arange of conversion ratios and output powers; reduces capacitor size,through reduction of the RMS capacitor current(s); and/or employssemiconductor power devices having reduced voltage ratings and betterperformance.

In various implementations, a modular DC-DC boost converter architectureemploys partial-power modules performing DC transformer (DCX), buck, andboost functions. These modules are able to operate with ultra-highefficiency over a restricted range of operating points, and are combinedinto a system architecture that performs the required DC-DC boostconversion function. The DCX module, for example, is able to perform anisolated boost function at a fixed conversion ratio, with very highefficiency. Boost and buck modules may operate with a restricted rangeof conversion ratios where their efficiency is very high and where thecapacitor current stresses and inductor applied ac voltages aresubstantially reduced. Voltage sharing between modules allows use oflower-voltage semiconductor devices having better characteristics, andalso reduces AC losses. One or more controllers may command theswitching of the semiconductor devices of the modules. Thesecontroller(s) may employ pass-through modes, in which one or moremodules simply connect their input and output ports to achieve aconversion ratio of unity; this improves efficiency by eliminating theAC loss of the module(s). The AC loss of the overall system is reduced,and hence the efficiency is increased over a range of output voltagesand powers.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, reference is made tothe following description and accompanying drawings, in which:

FIG. 1 is an example boost converter with unidirectional power flow;

FIG. 2 is an example boost converter with bidirectional power flow;

FIG. 3 is a typical plot of efficiency vs. duty cycle for a prior artboost converter;

FIG. 4 is a typical plot of efficiency vs. output power, for a fixedoutput voltage, for a prior art boost converter and also for an exampleimplementation of a boost converter provided herein;

FIG. 5 is an example three-level boost converter;

FIGS. 6 A and 6B include block diagrams of an example modular DC-DCconverters;

FIG. 7 is an representation of an example DC transformer module;

FIGS. 8 A and 8B and 8C include schematic diagrams of example DCtransformer modules;

FIG. 9 illustrates example DCX waveforms;

FIG. 10 is a representation of an example buck or boost converter modulehaving conversion ratio M(D);

FIG. 11 is a schematic of an example boost converter module withbidirectional power flow;

FIG. 12 illustrates a high efficiency operating point having a boostratio of 3;

FIG. 13 is a map of various example modes of operation of the DC-DCboost converter shown in FIG. 6B;

FIG. 14 illustrates example measured module waveforms for operation inthe DCX+Boost mode;

FIG. 15 illustrates an example implementation of control for a modularboost dc-dc system using a microcontroller and sensor signals;

FIG. 16 illustrates top-level blocks of an example implementation of amodular boost dc-dc system controller;

FIG. 17 shows an example relationship of a controller operating statusto operating modes of a modular boost system;

FIG. 18 is a detailed block diagram of an example main controller block,which implements average current control;

FIG. 19 is a detailed block diagram of an example DCX voltage limitblock;

FIG. 20 is a detailed block diagram of an example control mixer block;and

FIG. 21 is a detailed block diagram of an example control mixer block.

DETAILED DESCRIPTION

An example implementation of a modular DC-DC converter 100 architectureis illustrated in FIG. 6A. In this particular implementation, a DCtransformer module 102, DCX module 102, is provided in a stackedconfiguration with a DC-DC converter module 104 capable of operating ina pass-through mode. The DC-DC converter module 104 may, for example,include one or more of a Buck DC-DC converter module, a Boost DC-DCconverter module, a non-inverting Buck-Boost DC-DC converter module orany other type of converter module capable of being operated in apass-through mode.

The implementation of FIG. 6A includes the DCX module 102 and the DC-DCconverter module 104 arranged in such that the input ports 106, 108 ofthe DCX module 102 and the DC-DC converter module 104, respectively, areeach coupled to an input voltage V_(in). An output port 110 of the DCXmodule 102 is coupled in series with an output port 112 of the DC-DCconverter module 104. In this manner, an output voltage of the modularDC-DC converter 100 is a sum of the individual output voltagesV_(DCXout) of the DCX module 102 and V_(DC-DCCout) of the DC-DCconverter 104.

Another example implementation of a modular DC-DC boost converterarchitecture 120 is illustrated in FIG. 6B. With this approach, severalpartial-power converter modules 122, 124 and 126 are combined such thatthe system efficiency is optimized over a range of voltage and poweroperating points. By control of the modules 122, 124 and 126, theefficiency over the given range can be substantially higher than in aconventional full-power boost approach. In addition, the modules canshare the voltage stresses, allowing lower-voltage semiconductor devicesto be employed. Additionally, the choice of module types affects thesizes of the magnetic and capacitive elements of the system; thearchitecture allows the capacitive elements to be much smaller than incompeting approaches such as the multilevel boost architecture of FIG.5.

FIG. 6B illustrates another example modular dc-dc converter system 120including two DC-DC converter modules 122, 126 and a DC transformermodule 124. In this implementation, the first DC-DC converter module 122is a buck converter module 122, and the second DC-DC converter module126 is a boost converter module 126. This particular arrangement of theimplementation shown in FIG. 6B is, however, merely an exampleimplementation. The individual DC-DC converter modules (buck 122 andboost 126) or DC transformer module 124 may be arranged in other ordersor may include other types of modules. Each of the DC-DC convertermodules 122 and 126, for example, may include at least one or a buckconverter module, a boost converter module, a non-inverting buck-boostconverter module or any other type of DC-DC converter module capable ofoperating in a pass-through mode.

In the particular implementation shown in FIG. 6B, for example, thefirst DC-DC converter module 126 and the DC transformer module 124 arecoupled in series along a first, upper branch of the modular DC-DCconverter 120 and the second DC-DC converter module 126 is arrangedalong a second, lower branch of the modular DC-DC converter 120. Aninput port 128, in this case including two input terminals, of the firstDC-DC converter module 122 is coupled in parallel with an input port 136of the second DC-DC converter module 126 and the input ports are coupledto an input voltage Vin of the modular DC-DC converter 120. An outputport 130 of the first DC-DC converter module 122 is coupled to an inputport 132 of the DC transformer module 124. An output port 134 is coupledin series with an output port 138 of the second DC-DC converter module126. The output voltage Vout of the modular DC-DC converter module 120,thus, is a sum of the output voltages of the DC transformer module 124and the second DC-DC converter module 126.

In one implementation of a DC Transformer (DCX) module 140 illustratedin FIG. 7 is a zero-voltage switching DC-DC converter circuit containinga conventional (inductive) transformer. In this implementation, thiscircuit is optimized to achieve a very high efficiency at a singlevoltage conversion ratio V_(DCX-out)/V_(DCX-in)=N_(DCX). The input andoutput terminals are isolated. This type of circuit is generally notable to control its voltage conversion ratio. One well-known DCX circuitis the Dual Active Bridge converter 150, illustrated in FIG. 8. Thiscircuit 150 is capable of bidirectional power flow, and it achieveszero-voltage switching of the power semiconductors on the primary andsecondary sides of a transformer 154 through addition of a small tankinductor Ltank. The transformer physical turns ratio is approximatelythe same as the conversion ratio N_(DCX) defined above. In an exemplaryrealization, the DCX module of FIG. 6B is realized using the dual-activebridge circuit of FIG. 8, with typical waveforms as illustrated in FIG.9. This schematic illustrates use of power MOSFETs 152 in full-bridgeconfigurations on the primary and secondary sides of the transformer.These MOSFETs 152 include built-in body diodes, and are driven by gatedriver circuits 154. A DCX controller 158 produces control signals thatcommand the gate drivers to turn the MOSFETs 152 on and off. Each MOSFET152 conducts with a duty cycle of approximately 50%, except for a smalldead time inserted by the DCX controller to ensure that theseries-connected upper and lower MOSFETs 152 do not simultaneouslyconduct. FIG. 9 shows typical measured waveforms: the two upper tracesare gate drive signals for a pair of series-connected upper and lowerMOSFETs 152. Each MOSFET 152 conducts when its gate drive signal ishigh, and the dead time where both gate drive signals are low can beseen in the figure. Also shown in FIG. 9 (middle trace) is an exampletransformer primary winding current waveform. At a selected value ofoutput current, this transformer winding current waveform is nearlytrapezoidal, with peak value only slightly greater than the dc current.The lower trace in this example is the approximately 50% duty cyclevoltage waveform observed at the switch node between theseries-connected upper and lower MOSFETs. The voltage waveforms at theother three switch nodes are similar. For this discussion, it is assumedthat the DCX module operates with a conversion ratio that is fixed andequal to N_(DCX). Alternatively, the controller can turn the DCX moduleoff, with the input and output terminals operated as short circuits oras open circuits.

Other example implementations of DC transformer (DCX) transformermodules 160, 170 are illustrated in FIGS. 8B and 8C, respectively. Inthese implementations, the DCX modules are uni-directional DCX modulesin which either the input-side or output-side switches are replaced withdiodes D1 through D4 as shown in FIGS. 8B and 8C, respectively. In FIG.8B, for example, the output-side switches Q5 through Q8 of the dualactive bridge converter implementation shown in FIG. 8A are replacedwith diodes D1 through D4 and allow the DCX module of FIG. 8B to providepower flow from an input voltage V_(in) side to an output voltageV_(out) side of the DCX module. In the implementation of FIG. 8C,however, switches Q1 through Q4 of the dual active bridge converterimplementation shown in FIG. 8A are replaced with diodes D1 through D4.In this implementation, the DCX module provides power flow from anoutput voltage V_(out) side to an input voltage V_(in) side.

An example Buck or Boost PWM module 180 illustrated in FIG. 10 is aconventional hard-switched DC-DC converter. A schematic of an exampleboost converter module 190 is given in FIG. 11. In this example,semiconductor switches are realized using power MOSFET transistors Qt,Q2 having fast recovery body diodes, and power is able to flow in eitherdirection. A controller 192 generates pulse-width modulated (PWM) gatedrive signals to control the voltage conversion ratioM_(boost)(D)=V_(out)/V_(in)=1/(1−D) of this module, where D is the dutycycle of the lower MOSFET Q2. The upper MOSFET Q1 is driven with thecomplement of the gate drive signal applied to the lower MOSFET Q2, withthe exception of a small deadtime inserted into the gate drive signalsto ensure that the two MOSFETs Q1, Q2 do not simultaneously conduct. Inthis implementation, the buck module is identical to the boost module,but with the input and output terminals interchanged.

A buck or boost module achieves maximum efficiency in pass-through mode,where the conversion ratio is M(D)=1. This is achieved by causing thehigh-side semiconductor switch to remain in the on state: D=1 for thebuck converter, or D=0 for the boost converter. There is no switchingloss in the passthrough mode, and the input is connected to the outputthrough the high-side semiconductor device and the inductor. Very highefficiencies are achieved in passthrough mode. Neighboring operatingpoints, with pulse-width modulation at a duty cycle D near thepassthrough value, also achieves high efficiency but with some switchingloss. Operation at PWM duty cycles farther from the passthrough value isundesirable because of the increased ac losses in the magnetics,increased ac capacitor currents, and overall reduced efficiency.

To appreciate how the modular boost architecture can improve theefficiency at high output bus voltages, consider the operating pointillustrated in FIG. 12. For this example, an overall system conversionratio is V_(out)/V_(in)=3. If a single boost converter were employed inthis example, it would operate with a high duty cycle of over 0.7, andwould exhibit relatively low efficiency. However, in the modular boostconfiguration 200, with N_(DCX)=2, the buck and boost modules operate inpassthrough mode with very high efficiency. The DCX output voltage ofN_(DCX)V_(in) is added to the boost output voltage of V_(in), leading toa total dc output voltage of (1+N_(DCX))V_(in)=3V_(in). The systemefficiency is very high at this operating point because the buck andboost modules operate in pass-through mode, and the DCX operates withvery high efficiency at its optimized conversion ratio of 2. The dcoutput voltage can be increased by increasing the boost duty cycle, andthe dc output voltage can be decreased by decreasing the buck dutycycle.

FIG. 13 summarizes how the modules may be controlled. In this example,it is assumed that the input voltage can vary over the rangeV_(in,min)≦V_(in)≦V_(in,max), and the output voltage is controlled overthe range V_(in)≦V_(out)≦V_(out,max). A maximum conversion ratio M_(max)is also assumed. Additionally, the operation modes of FIG. 13 assumethat the semiconductor device voltages are constrained to be no greaterthan V_(Q,max) and this limit is taken to be the maximum allowed voltageat the input or output port of any module. FIG. 13 illustrates fourexample system operating modes. In the region labeled “DCX+Boost”, thebuck converter operates in pass-through mode while the boost converteroperates with PWM at D>0. The DCX operates, with fixed conversion ratioN_(DCX). In the region labeled “DCX+Buck”, the boost converter operatesin pass-through mode, while the buck converter operates with PWM at D<1.In the region labeled “DCX Buck Boost”, both the buck and the boostconverters operate with PWM away from pass-through mode: the buckconverter limits its output voltage to approximately V_(Q,max)/N_(DCX),so that the voltage at the DCX output does not exceed V_(Q,max). Theboost converter produces a voltage of (V_(out)−V_(Q,max)). Finally, forthe region labeled “Boost only”, the DCX is shut down, with itsoutput-side switches turned on so that the DCX output voltage is zero.The boost converter operates with PWM to produce the required outputvoltage. Control to implement this strategy is described in a latersection of this disclosure.

Thus, in various boost implementations, a modular boost architectureallows substantial improvement of system efficiency over a range ofoperating points that require substantial boosting of the voltage. Thesystem architecture causes the output dc voltage to be shared betweenthe series-connected DCX and boost modules, reducing the voltage appliedto any individual semiconductor switch. Hence, the modules can employlower-voltage-rated devices having better characteristics. Because ofthe reduced module voltage and parallel input current paths, theindividual modules are rated at a fraction of the total system outputpower.

Example Controller Architecture

A possible control objective in the modular boost dc-dc converter systemis to regulate the dc output voltage V_(out) at a reference value setwithin a controller. In this implementation, the dc output voltage is tobe regulated against static and dynamic variations in input voltageV_(in) and the output load current. In a conventional single dc-dcconverter system, the voltage regulation feedback loop is relativelysimple: the output voltage is sensed and compared to a reference; theerror between the sensed bus voltage and the reference is processed by acontroller to determine the converter control signal, i.e., the dutycycle of the converter power switches. In a modular dc-dc boost system,the control loop is more complex since power is processed by two, threeor more interconnected converter modules. In the implementation of FIG.6B, for example, one or more controllers control a controller a boostDC-DC converter module, a buck DC-DC converter module, and a DCX module.In one implementation, such as the converter shown in FIG. 6B forexample, control signals for the three modules are as follows:

-   -   Buck module: buck converter PWM signal with duty cycle d_(buck),        0≦d_(buck)≦1, where d_(buck)=0 in the shut-down mode, and        d_(buck)=1 in the pass-through mode;    -   Boost module: boost converter PWM signal with duty cycle        d_(boost), 0≦d_(boost)<1, where d_(boost)=0 in the boost        pass-through mode;    -   DCX module: DCX enable signal that indicates if the DCX module        is active or shut down.        The modular dc-dc boost controller determines and coordinates        the module control signals in response to the voltage error        signal, as well as in response to the location of the system        operating point in the space of input and output voltages.        Determination and coordination of the module control signals can        be accomplished in a number of ways. This section summarizes a        centralized modular dc-dc boost system controller 210        architecture, having the high-level structure illustrated in        FIG. 15. Sensed signals (module voltages and currents) are        received by a single central controller, in this example,        implemented in a microcontroller chip. This microcontroller        includes analog-to-digital converters (ADCs) that receive the        sensed signals, digital implementation of control algorithms,        and PWM outputs that provide the PWM signals listed above: the        buck converter PWM signals, boost converter PWM signals, and the        DCX module PWM signals including primary and secondary PWM        signals that are generated based on the DCX enable signal.

FIG. 16 shows a block diagram of one implementation of a controllerarchitecture 220 that illustrates how the module control signals aregenerated based on the sensed power-stage signals and the output voltagereference v_(out,ref). The “voltage/current limiter” block limitsadjusts the output voltage reference value so that it always stayswithin the expected range. Furthermore, the output voltage reference canbe adjusted if a sensed steady-state inductor current is too high, thusproviding an over-current protection function. Based on the sensedoutput voltage, the reference output voltage, and the sensed inductorcurrents, the “main controller” determines a single control variablecalled d_(control). The main controller includes a dynamic responsedesigned to ensure stability and well-behaved transient responses of theclosed-loop controlled system. By design, the main control variabled_(control) is between 0 and 2.

The purpose of the “DCX voltage limiter” is to ensure that the DCXoutput voltage stays below a set value V_(Q,max). The output of the DCXvoltage limiter is an auxiliary control variable d_(limit), which isbetween 0 and 1.

The “boost only” block decides if the buck module should be turned on ornot depending on the requested bus voltage. The “boost only” blockoutputs an auxiliary control variable d_(bk) _(—) _(shn), which isbetween 0 and 1.

The “control mixer” takes the main control variable d_(control), and thetwo auxiliary control variables, d_(limit) and d_(bk) _(—) _(shn), anddetermines the module control signals d_(buck) (buck module duty cycle),d_(boost)(boost module duty cycle) and DCX_en (DCX enable).

FIG. 17 shows how the controller operating status is related to themodular boost system operating mode. In the Boost mode, the “boost only”block shuts down the buck and the DCX modules, and the voltageregulation loop is closed through the main controller. In theDCX+Buck+Boost mode, the “DCX voltage limiter” block regulates the DCXoutput to V_(Q,max), and the “main controller” regulates the outputvoltage. In the DCX+Buck or DCX+Boost modes, the “main controller”regulates the output voltage. The value of the main control variabled_(control) decides if the system is in the DCX+boost mode(1<d_(control)<2), or in the DCX+buck mode (0<d_(control)<1).

A more detailed block diagram of a “main controller” 230 is shown inFIG. 18. This is a standard average current mode control with an outervoltage loop including the voltage-loop compensator G_(cv) and an innercurrent loop including the current-loop compensator G_(ci). The innercurrent loop is setup to regulate the average of the buck and the boostinductor currents. The compensators G_(ci) and G_(ev) are simpleproportional-plus-integral (PI) compensators. The range of i_(ref) islimited so that the dynamic range of the average inductor current islimited. The main control variable d_(control) is by design limitedbetween 0 and 2.

A more detailed block diagram of a “DCX voltage limit” block 240 isshown in FIG. 19. The DCX output voltage v_(Dcx) is obtained bysubtracting the sensed boost output voltage from the sensed output busvoltage. v_(Dcx) is compared to a reference value of V_(Q,max), denotedV_(Q,max-ref), and the error is processed by G_(climit), which is aproportional-plus-integral-plus-derivative (PID) compensator producingthe auxiliary control variable d_(limit), which is between 0 and 1. Ifthe DCX output voltage is less than V_(Q,max-ref), the auxiliary controlvariable d_(limit) saturates to 1. If the DCX output exceeds 400 V(which may occur in the DCX+Buck+Boost mode), the auxiliary controlvariable d_(limit) becomes less than 1.

A “boost only” block is implemented as an up/down counter. When therequested output voltage reference is greater than V_(Q,max-ref), itcounts down from 1 to 0; when the requested output voltage is less thanV_(Q,max-ref), it counts up from 0 to 1.

A detailed design of a “control mixer” block 250 is shown in FIG. 20.The buck module control variable (duty cycle d_(buck)) is obtained by:

1. Taking the minimum of d_(control) and d_(limit);

2. Subtracting d_(bk) _(—) _(shn) to get d′_(buck);

3. Limiting the final d_(buck) between 0 and 1

If the DCX output is greater than V_(Q,max-ref), then d_(limit) drops toa value smaller than d_(control), so that d_(buck) is determined byd_(limit). In this case, the buck module is effectively controlled tolimit the DCX output voltage to V_(Q,max-ref). If the DCX output voltageis less than V_(Q,max-ref), then d_(limit)=1. In this case, ifd_(control)<1, d_(buck) is determined by d_(control), which means thatthe buck module regulates the system output voltage. This occurs in theDCX+Buck mode. If d_(control)>1, then d_(buck)=1, and the buck module isin the pass-through mode. This occurs in the DCX+Boost mode. If therequested bus voltage is less than V_(Q,max-ref), then d_(bk) _(—)_(shn), gradually counts from 0 to 1, which ultimately leads tod′_(buck)≦0, and the resulting d_(bk)=0 shuts down the buck module. Thisoccurs in the Boost mode.

The DCX control signal DCX_(en) shuts down the DCX module wheneverd′_(buck)≦0. The boost module control variable d_(boost) obtained bypassing d_(control)−1, through a 0 to 1 limiter. When d_(control)<1,then d_(boost)=0, which means that the boost module in the pass-throughmode. This occurs in the system DCX+buck model. Otherwise the boostmodule regulates the bus voltage.

With the control approach described above, all mode-changing decisionsare based on values of the main and the auxiliary control variables, andall mode transitions are smooth and occur according to the internalstate of each controller block. However, with the “control mixer” designin FIG. 20, one potential problem is that when the buck or the boostmodule enters its pass-through or shut-down mode, the module is nolonger controlled unless the input or the output voltage changes. As aresult, upon transition to pass-through or shut-down mode the inductorcurrent may ring at the power stage natural frequency. To address thisissue, the sensed buck and the boost inductor currents can be added tothe “control mixer” as shown in in FIG. 21. The band-pass filtersG_(BPF) pass only the components of the sensed current around thenatural frequency of the power stage. In response, the controller willrespond to and suppress a ringing disturbance in the correspondinginductor current.

Many implementations have been described with reference to theaccompanying Figures. Various features introduced in particularimplementations are also intended to be used in other implementationswhere appropriate. For example, while one or more features may bedescribed with respect to one or more particular implementations, thisis merely for convenience and is not intended to imply that thosefeatures are only contemplated to be used in that particularimplementation.

Although many implementations have been described above with a certaindegree of particularity, those skilled in the art could make numerousalterations to the disclosed embodiments without departing from thespirit or scope of this invention. The number and type of modules (e.g.,DC-DC converter modules, DC transformer modules) in variousimplementations of a modular DC-DC converter could be varied as well astheir arrangement within the modular DC-DC converter. For example,although the implementation of FIG. 6B describes a boost modular DC-DCconverter, the implementation is not limited to just boosconfigurations. Any other DC-DC converter module capable of operating ina pass-through mode may be used instead of another particular DC-DCconverter module. Similarly, various DC transformer modules may be usedinstead of the particular DC transformer modules described herein.

All directional references (e.g., upper, lower, upward, downward, left,right, leftward, rightward, top, bottom, above, below, vertical,horizontal, clockwise, and counterclockwise) are only used foridentification purposes to aid the readeras understanding of the presentinvention, and do not create limitations, particularly as to theposition, orientation, or use of the invention. Joinder references(e.g., attached, coupled, connected, and the like) are to be construedbroadly and may include intermediate members between a connection ofelements and relative movement between elements. As such, joinderreferences do not necessarily infer that two elements are directlyconnected and in fixed relation to each other. It is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative only and not limiting.Changes in detail or structure may be made without departing from thespirit of the invention as defined in the appended claims

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed:
 1. A modular dc-dc converter system, comprising: adc-dc converter module including a converter module input port and aconverter module output port; a dc transformer module including atransformer module input port and a transformer module output port; acontroller that supplies one or more duty cycle signals to at least oneswitch of the dc-dc converter module and one or more control signals tothe dc transformer module; a system input voltage port; and a systemoutput voltage port, wherein the converter module input port of thedc-dc converter module is coupled to the system input voltage port, theinput port of the dc transformer module is coupled to the system inputvoltage port, the output port of the dc-dc converter module is coupledin series with the output port of the dc transformer module and theseries-coupled ports are coupled to the system output voltage port. 2.The apparatus of claim 1 wherein the controller commands the dc-dcconverter to operate in a pass-through mode over a range of input andoutput voltages.
 3. The apparatus of claim 1 wherein the controllercomprises a microcontroller.
 4. The apparatus of claim 1 wherein thetransformer output port of the dc transformer module and the convertermodule output port of the dc-dc converter module share voltage stressesof the modular dc-dc converter system.
 5. The apparatus of claim 1wherein the dc transformer module comprises a zero-voltage switchingdc-dc converter circuit.
 6. The apparatus of claim 1 wherein the dc-dcconverter module comprises a hard-switched dc-dc converter module. 7.The apparatus of claim 1 wherein a second dc-dc converter module iscoupled in series between the transformer module input port and thesystem input voltage port.
 8. The apparatus of claim 7 wherein a secondconverter module input port of the second dc-dc converter module iscoupled in parallel with the converter module input port of the dc-dcconverter module and a second converter module output port of the seconddc-dc converter module is coupled in series with the transformer moduleinput port.
 9. A modular dc-dc converter system, comprising: a firstdc-dc converter module, including a first converter module input portand a first converter module output port; a second dc-dc convertermodule, including a second converter module input port and a secondconverter module output port; a dc transformer module, including atransformer module input port and a transformer module output port; acontroller that supplies at least one first duty cycle signal to atleast one switch of the first dc-dc converter module, at least onesecond duty cycle signal to at least one switch of the second dc-dcconverter module, and one or more control signals to the dc transformermodule; a system input voltage port; and a system output voltage port,wherein the first converter module input port of the first dc-dcconverter module and the second converter module input port of thesecond dc-dc converter module are coupled connected in parallel, andboth the first dc-dc converter module and the second dc-dc convertermodule are coupled to the system input voltage port; the first convertermodule output port of the first dc-dc converter module is coupled to thetransformer module input port of the dc transformer module; the secondconverter module output port of the second dc-dc converter module iscoupled in series with the output port of the dc transformer module, andthe series-coupled ports are coupled to the system output voltage port.10. The apparatus of claim 9 wherein the controller is configured tocommand the first dc-dc converter module to operate in pass-through modeover a range of input and output voltages.
 11. The apparatus of claim 9wherein the controller is configured to command the second dc-dcconverter module to operate in pass-through mode over a range of inputand output voltages
 12. The apparatus of claim 9 wherein the controllercomprises a microcontroller.
 13. The apparatus of claim 9 wherein thetransformer output port of the dc transformer module and the secondconverter module output port of the second dc-dc converter module sharevoltage stresses of the modular dc-dc converter system.
 14. Theapparatus of claim 9 wherein the dc transformer module comprises azero-voltage switching dc-dc converter circuit.
 15. The apparatus ofclaim 14 wherein the dc transformer module includes an inductivetransformer.
 16. The apparatus of claim 14 wherein the dc transformermodule comprises isolated input and output terminals.
 17. The apparatusof claim 9 wherein each of the first dc-dc converter module and thesecond dc-dc converter module comprises a hard-switched dc-dc convertermodule.
 18. The apparatus of claim 9 wherein each of the first dc-dcconverter module and the second dc-dc converter module comprise at leastone of a buck dc-dc converter module, a boost dc-dc converter module anda non-inverting buck-boost dc-dc converter module.
 19. A method ofcontrolling a modular dc-dc converter system, the method comprising:providing a modular dc-dc converter system comprising: a dc-dc convertermodule including a converter input port and a converter output port; adc transformer module including a transformer input port and atransformer output port; a controller that supplies at least one dutycycle signal to at least one switch of the dc-dc converter module and atleast one control signal to the dc transformer module; a system inputvoltage port; and a system output voltage port, wherein the converterinput port of the dc-dc converter module and the transformer input portof the dc transformer module are coupled to the system input voltageport, the converter output port of the dc-dc converter module is coupledin series with the transformer output port of the dc transformer moduleand the series-coupled ports are coupled to the system output voltageport; and in a first operational state, controlling the dc-dc convertermodule to operate with pulse-width modulation (PWM) and controlling thedc transformer module with a fixed conversion ratio; and in a secondoperational state, performing an operation comprising at least one of(i) controlling the dc-dc converter module to operate in a pass-throughmode and (ii) controlling the dc transformer module to shut down andcontrolling the dc-dc converter module to operate with PWM.
 20. Themethod of claim 19 wherein the operation performed in the secondoperational state comprises controlling the dc-dc converter module tooperate in a pass-through mode and, in a third operational state,performing an operation of controlling the dc transformer module to shutdown and controlling the dc-dc converter module to operate with PWM. 21.The method of claim 20 wherein, the dc transformer module is controlledto shut down by turning on a plurality of dc transformer module outputswitches.
 22. A method of controlling a modular dc-dc converter system,the method comprising: providing a modular dc-dc converter systemcomprising: a first dc-dc converter module including a first converterinput port and a first converter output port; a second dc-dc convertermodule including a second converter input port and a second converteroutput port; a dc transformer module including a transformer input portand a transformer output port; a controller that supplies at least onefirst duty cycle signal to at least one switch of the first convertermodule, at least one second duty cycle signal to at least one switch ofthe second converter module, and one or more control signals to the dctransformer module; a system input voltage port; and a system outputvoltage port, wherein the first converter input port of the first dc-dcconverter module and the second converter input port of the second dc-dcconverter module are coupled in parallel, both the first dc-dc convertermodule and the second dc-dc converter module are coupled to the systeminput voltage port; the first converter output port of the first dc-dcconverter module is coupled to the transformer input port of the dctransformer module; the second converter output port of the second dc-dcconverter module is coupled in series with the transformer output portof the dc transformer module, and the series-coupled ports are coupledto the system output voltage port; and in a first operational state,controlling the dc transformer module to shut down with dc transformermodule output switches turned on and controlling the second dc-dcconverter module to operate with PWM; and in a second operational stateperforming an operation comprising at least one of (i) controlling thefirst dc-dc converter module to operate in a pass-through mode,controlling the second dc-dc converter module to operate withpulse-width modulation (PWM) and controlling the dc transformer modulewith a fixed conversion ratio and (ii) controlling the second dc-dcconverter module to operate in a pass-through mode and controlling thefirst dc-dc converter module to operate with PWM and controlling the dctransformer module with a fixed conversion ratio.
 23. The method ofclaim 22 wherein, in a third operational state, in a third operationalstate, controlling the first dc-dc converter module and the second dc-dcconverter module to operate with PWM, wherein the output voltage of thesecond dc-dc converter module is limited.
 24. The method of claim 22wherein, in a third operational state, controlling the first and seconddc-dc converter modules to operate in a pass-through mode andcontrolling the dc transformer module with a fixed conversion ratio.